/*-------------------------------------------------------------------------

 *

 * bgwriter.c

 *

 * The background writer (bgwriter) is new as of Postgres 8.0.  It attempts

 * to keep regular backends from having to write out dirty shared buffers

 * (which they would only do when needing to free a shared buffer to read in

 * another page).  In the best scenario all writes from shared buffers will

 * be issued by the background writer process.  However, regular backends are

 * still empowered to issue writes if the bgwriter fails to maintain enough

 * clean shared buffers.

 *

 * As of Postgres 9.2 the bgwriter no longer handles checkpoints.

 *

 * Normal termination is by SIGTERM, which instructs the bgwriter to exit(0).

 * Emergency termination is by SIGQUIT; like any backend, the bgwriter will

 * simply abort and exit on SIGQUIT.

 *

 * If the bgwriter exits unexpectedly, the postmaster treats that the same

 * as a backend crash: shared memory may be corrupted, so remaining backends

 * should be killed by SIGQUIT and then a recovery cycle started.

 *

 *

 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group

 *

 *

 * IDENTIFICATION

 *    src/backend/postmaster/bgwriter.c

 *

 *-------------------------------------------------------------------------

 */

#include "postgres.h"

#include "access/xlog.h"

#include "libpq/pqsignal.h"

#include "miscadmin.h"

#include "pgstat.h"

#include "postmaster/auxprocess.h"

#include "postmaster/bgwriter.h"

#include "postmaster/interrupt.h"

#include "storage/aio_subsys.h"

#include "storage/buf_internals.h"

#include "storage/bufmgr.h"

#include "storage/condition_variable.h"

#include "storage/fd.h"

#include "storage/lwlock.h"

#include "storage/proc.h"

#include "storage/procsignal.h"

#include "storage/smgr.h"

#include "storage/standby.h"

#include "utils/memutils.h"

#include "utils/resowner.h"

#include "utils/timestamp.h"

/*

 * GUC parameters

 */

int     BgWriterDelay = 200;

/*

 * Multiplier to apply to BgWriterDelay when we decide to hibernate.

 * (Perhaps this needs to be configurable?)

 */

#define HIBERNATE_FACTOR      50

/*

 * Interval in which standby snapshots are logged into the WAL stream, in

 * milliseconds.

 */

#define LOG_SNAPSHOT_INTERVAL_MS 15000

/*

 * LSN and timestamp at which we last issued a LogStandbySnapshot(), to avoid

 * doing so too often or repeatedly if there has been no other write activity

 * in the system.

 */

static TimestampTz last_snapshot_ts;

static XLogRecPtr last_snapshot_lsn = InvalidXLogRecPtr;

/*

 * Main entry point for bgwriter process

 *

 * This is invoked from AuxiliaryProcessMain, which has already created the

 * basic execution environment, but not enabled signals yet.

 */

void

BackgroundWriterMain(const void *startup_data, size_t startup_data_len)

{

  sigjmp_buf  local_sigjmp_buf;

  MemoryContext bgwriter_context;

  bool    prev_hibernate;

  WritebackContext wb_context;

  Assert(startup_data_len == 0);

  MyBackendType = B_BG_WRITER;

  AuxiliaryProcessMainCommon();

  /*

   * Properly accept or ignore signals that might be sent to us.

   */

  pqsignal(SIGHUP, SignalHandlerForConfigReload);

  pqsignal(SIGINT, SIG_IGN);

  pqsignal(SIGTERM, SignalHandlerForShutdownRequest);

  /* SIGQUIT handler was already set up by InitPostmasterChild */

  pqsignal(SIGALRM, SIG_IGN);

  pqsignal(SIGPIPE, SIG_IGN);

  pqsignal(SIGUSR1, procsignal_sigusr1_handler);

  pqsignal(SIGUSR2, SIG_IGN);

  /*

   * Reset some signals that are accepted by postmaster but not here

   */

  pqsignal(SIGCHLD, SIG_DFL);

  /*

   * We just started, assume there has been either a shutdown or

   * end-of-recovery snapshot.

   */

  last_snapshot_ts = GetCurrentTimestamp();

  /*

   * Create a memory context that we will do all our work in.  We do this so

   * that we can reset the context during error recovery and thereby avoid

   * possible memory leaks.  Formerly this code just ran in

   * TopMemoryContext, but resetting that would be a really bad idea.

   */

  bgwriter_context = AllocSetContextCreate(TopMemoryContext,

                       "Background Writer",

                       ALLOCSET_DEFAULT_SIZES);

  MemoryContextSwitchTo(bgwriter_context);

  WritebackContextInit(&wb_context, &bgwriter_flush_after);

  /*

   * If an exception is encountered, processing resumes here.

   *

   * You might wonder why this isn't coded as an infinite loop around a

   * PG_TRY construct.  The reason is that this is the bottom of the

   * exception stack, and so with PG_TRY there would be no exception handler

   * in force at all during the CATCH part.  By leaving the outermost setjmp

   * always active, we have at least some chance of recovering from an error

   * during error recovery.  (If we get into an infinite loop thereby, it

   * will soon be stopped by overflow of elog.c's internal state stack.)

   *

   * Note that we use sigsetjmp(..., 1), so that the prevailing signal mask

   * (to wit, BlockSig) will be restored when longjmp'ing to here.  Thus,

   * signals other than SIGQUIT will be blocked until we complete error

   * recovery.  It might seem that this policy makes the HOLD_INTERRUPTS()

   * call redundant, but it is not since InterruptPending might be set

   * already.

   */

  if (sigsetjmp(local_sigjmp_buf, 1) != 0)

  {

    /* Since not using PG_TRY, must reset error stack by hand */

    error_context_stack = NULL;

    /* Prevent interrupts while cleaning up */

    HOLD_INTERRUPTS();

    /* Report the error to the server log */

    EmitErrorReport();

    /*

     * These operations are really just a minimal subset of

     * AbortTransaction().  We don't have very many resources to worry

     * about in bgwriter, but we do have LWLocks, buffers, and temp files.

     */

    LWLockReleaseAll();

    ConditionVariableCancelSleep();

    pgaio_error_cleanup();

    UnlockBuffers();

    ReleaseAuxProcessResources(false);

    AtEOXact_Buffers(false);

    AtEOXact_SMgr();

    AtEOXact_Files(false);

    AtEOXact_HashTables(false);

    /*

     * Now return to normal top-level context and clear ErrorContext for

     * next time.

     */

    MemoryContextSwitchTo(bgwriter_context);

    FlushErrorState();

    /* Flush any leaked data in the top-level context */

    MemoryContextReset(bgwriter_context);

    /* re-initialize to avoid repeated errors causing problems */

    WritebackContextInit(&wb_context, &bgwriter_flush_after);

    /* Now we can allow interrupts again */

    RESUME_INTERRUPTS();

    /*

     * Sleep at least 1 second after any error.  A write error is likely

     * to be repeated, and we don't want to be filling the error logs as

     * fast as we can.

     */

    pg_usleep(1000000L);

    /* Report wait end here, when there is no further possibility of wait */

    pgstat_report_wait_end();

  }

  /* We can now handle ereport(ERROR) */

  PG_exception_stack = &local_sigjmp_buf;

  /*

   * Unblock signals (they were blocked when the postmaster forked us)

   */

  sigprocmask(SIG_SETMASK, &UnBlockSig, NULL);

  /*

   * Reset hibernation state after any error.

   */

  prev_hibernate = false;

  /*

   * Loop forever

   */

  for (;;)

  {

    bool    can_hibernate;

    int     rc;

    /* Clear any already-pending wakeups */

    ResetLatch(MyLatch);

    ProcessMainLoopInterrupts();

    /*

     * Do one cycle of dirty-buffer writing.

     */

    can_hibernate = BgBufferSync(&wb_context);

    /* Report pending statistics to the cumulative stats system */

    pgstat_report_bgwriter();

    pgstat_report_wal(true);

    if (FirstCallSinceLastCheckpoint())

    {

      /*

       * After any checkpoint, free all smgr objects.  Otherwise we

       * would never do so for dropped relations, as the bgwriter does

       * not process shared invalidation messages or call

       * AtEOXact_SMgr().

       */

      smgrdestroyall();

    }

    /*

     * Log a new xl_running_xacts every now and then so replication can

     * get into a consistent state faster (think of suboverflowed

     * snapshots) and clean up resources (locks, KnownXids*) more

     * frequently. The costs of this are relatively low, so doing it 4

     * times (LOG_SNAPSHOT_INTERVAL_MS) a minute seems fine.

     *

     * We assume the interval for writing xl_running_xacts is

     * significantly bigger than BgWriterDelay, so we don't complicate the

     * overall timeout handling but just assume we're going to get called

     * often enough even if hibernation mode is active. It's not that

     * important that LOG_SNAPSHOT_INTERVAL_MS is met strictly. To make

     * sure we're not waking the disk up unnecessarily on an idle system

     * we check whether there has been any WAL inserted since the last

     * time we've logged a running xacts.

     *

     * We do this logging in the bgwriter as it is the only process that

     * is run regularly and returns to its mainloop all the time. E.g.

     * Checkpointer, when active, is barely ever in its mainloop and thus

     * makes it hard to log regularly.

     */

    if (XLogStandbyInfoActive() && !RecoveryInProgress())

    {

      TimestampTz timeout = 0;

      TimestampTz now = GetCurrentTimestamp();

      timeout = TimestampTzPlusMilliseconds(last_snapshot_ts,

                          LOG_SNAPSHOT_INTERVAL_MS);

      /*

       * Only log if enough time has passed and interesting records have

       * been inserted since the last snapshot.  Have to compare with <=

       * instead of < because GetLastImportantRecPtr() points at the

       * start of a record, whereas last_snapshot_lsn points just past

       * the end of the record.

       */

      if (now >= timeout &&

        last_snapshot_lsn <= GetLastImportantRecPtr())

      {

        last_snapshot_lsn = LogStandbySnapshot();

        last_snapshot_ts = now;

      }

    }

    /*

     * Sleep until we are signaled or BgWriterDelay has elapsed.

     *

     * Note: the feedback control loop in BgBufferSync() expects that we

     * will call it every BgWriterDelay msec.  While it's not critical for

     * correctness that that be exact, the feedback loop might misbehave

     * if we stray too far from that.  Hence, avoid loading this process

     * down with latch events that are likely to happen frequently during

     * normal operation.

     */

    rc = WaitLatch(MyLatch,

             WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,

             BgWriterDelay /* ms */ , WAIT_EVENT_BGWRITER_MAIN);

    /*

     * If no latch event and BgBufferSync says nothing's happening, extend

     * the sleep in "hibernation" mode, where we sleep for much longer

     * than bgwriter_delay says.  Fewer wakeups save electricity.  When a

     * backend starts using buffers again, it will wake us up by setting

     * our latch.  Because the extra sleep will persist only as long as no

     * buffer allocations happen, this should not distort the behavior of

     * BgBufferSync's control loop too badly; essentially, it will think

     * that the system-wide idle interval didn't exist.

     *

     * There is a race condition here, in that a backend might allocate a

     * buffer between the time BgBufferSync saw the alloc count as zero

     * and the time we call StrategyNotifyBgWriter.  While it's not

     * critical that we not hibernate anyway, we try to reduce the odds of

     * that by only hibernating when BgBufferSync says nothing's happening

     * for two consecutive cycles.  Also, we mitigate any possible

     * consequences of a missed wakeup by not hibernating forever.

     */

    if (rc == WL_TIMEOUT && can_hibernate && prev_hibernate)

    {

      /* Ask for notification at next buffer allocation */

      StrategyNotifyBgWriter(MyProcNumber);

      /* Sleep ... */

      (void) WaitLatch(MyLatch,

               WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,

               BgWriterDelay * HIBERNATE_FACTOR,

               WAIT_EVENT_BGWRITER_HIBERNATE);

      /* Reset the notification request in case we timed out */

      StrategyNotifyBgWriter(-1);

    }

    prev_hibernate = can_hibernate;

  }

}